A redditor who no longer wishes to be pinged has previously answered When did the fundamental differences between Sunni, Shiites and others develop? What were the catalysts for these differences?

/u/frogbrooks has previously answered When the Shias and Sunnis originally split, was it because of political reasons, or religious reasons?

/u/yodatsracist talks about this in Was the Christian world, particularly around the Crusades, aware of the schism between Sunni and Shia Islam? and probably elsewhere on /r/AskHistorians.